A student has begun training for tryouts for lacrosse by running a two
mile loop through her neighborhood every evening. Part of the run is
a very steep hill. One evening, the student is running down the hill at
a very quick pace when she notices a dog lying in the middle of her
path. To avoid hitting the dog, she tries to stop but has difficulty. With
effort, she is able to stop right before running into the dog. Why is it
so difficult for the student to stop?